Determining where an object is located in an urban environment can be quite challenging, particularly where the object is located in a multi-story building. Imprecise estimates of a person's altitude, for example, can delay emergency response times, forcing emergency response units to search several floors instead of one floor. Identifying the wrong floor level at which a person resides can also disrupt efforts to restrict, grant or track that person's access to rooms or information based on the location of that person. Of course, there are many other reasons why an accurate estimate of the location of a person or other object at different altitudes is desirable.
Various approaches have been used to estimate the altitude at which an object resides. For example, one common technique uses estimated distances between the object and various transmitters at known locations, where the estimated distances are determined by means of positioning signals sent from those transmitters to the object. Such transmitters may include orbiting satellites of a Global Navigation Satellite System (GNSS) like the Global Positioning System (GPS), or the transmitters include beacons in a terrestrial network of beacons that are dispersed throughout the urban environment.
Positioning signals from beacons in a terrestrial network are usually adequate for estimating the altitude of an object so long as the object has an unobstructed view of the terrestrial beacons and so long as the beacons are installed at different altitudes. Unfortunately, positioning signals from terrestrial beacons are frequently obstructed by buildings that stand between the beacons and the object. The presence of obstructions like buildings leads to unreliable estimates of the distances between the beacons and the object, which decreases the accuracy of the object's estimated altitude when using those unreliable estimated distances.
Other approaches for estimating the altitude of an object use environmental conditions like pressure and temperature. For example, in co-assigned U.S. application Ser. No. 13/296,067, Wide Area Positioning Systems (Nov. 14, 2011), pressure and temperature information collected at one or more reference locations in an urban environment may be used along with pressure collected by the object to determine an altitude of the object. In dense urban settings with buildings that obstruct positioning signals from beacons, use of environmental conditions like pressure and temperature can provide more-accurate estimates of altitude than using positioning signals. Under some conditions, like humid conditions experienced in many densely populated cities throughout the world, the presence of humidity can decrease the accuracy of an estimated altitude that was computed using pressure and temperature without compensating for the effect that humidity has on pressure. Thus, new and improved approaches that use pressure and temperature to estimate the altitude of an object while compensating for the effect humidity has on pressure are needed.